Ether alcohols, such as 2-butoxyethanol, have important industrial functions in such products such as cleaning supplies and coating materials. In the past, the manufacture of these products has been based on a process relying on a reaction between an alcohol and ethylene oxide. This conventional process has proven to be somewhat inefficient, in that it produces various undesirable byproducts along with the ether alcohols.
Monoether glycols can also be manufactured in a process dependent on aldehydes and a polyhydroxyl compound, instead of ethylene oxide, as starting materials. In such processes cyclic acetals are first generated. The acetal of ethylene glycol and butyraldehyde, for example, is described by Hibbert and Timm (Hibbert, H.; Timm, J. A. J. Am. Chem. Soc. 1924, 46(5), 1283-1290.) and is achieved with a maximum yield of 50% under acidic conditions. These cyclic acetals can then be subjected to hydrogenolysis in the presence of palladium and phosphoric acid catalysts. Such a process is described in U.S. Pat. No. 4,484,009.
The reaction of the polyhydroxyl compounds with aldehydes is an equilibrium reaction with the acetal product and co-product water. Yield of acetal is reduced via hydrolysis of the acetal by the co-product water. Thus, it is desirable to remove water from the reaction system to increase yield of the acetal.
The separation of water from the reaction mixture has been difficult since it often forms an azeotrope with the aldehyde reactants and with the cyclic acetal products. Entrainers have been employed to remove water through azeotropic distillation. Sulzbacher and coworkers, for example, describe removing the water by using benzene during the preparation of a number of acetals of ethylene glycol (Sulzbacher, M. et. al. J. Am. Chem. Soc. 1948, 70(8), 2827-2828). The environmental and health impact of benzene is an obvious concern in this method. Desiccants such as calcium chloride (DE 419223; Brönsted and Grove J. Am. Chem. Soc. 1930, 52(4), 1394-1403) may be employed in the reaction vessel to remove water as it is formed, but disposal of the generated solid waste is an economic and environmental concern.
Another method as described by Astle and coworkers, involves heating the glycol and aldehyde in a batch process over an heterogeneous acidic resin and distilling out in an overhead stream the acetal and water as they are formed (Astle, M. J. et al, Ind. Eng. Chem. 1954, 46(4), 787-791). This method generates high yields, but catalyst lifetime, cost, and the economic impact of multiple reaction vessels is a drawback.
In U.S. Pat. No. 5,917,059 to BASF Aktiengesellschaft, the authors generated cyclic acetals and ketals by reacting a molar excess of aldehydes and ketones with polyhydroxyl compounds in the presence of an acid catalyst. The water is removed by continuously distilling unreacted aldehydes or ketone starting materials, thus co-distilling the formed water in the water/aldehyde azeotrope, and further replacing the distilled aldehyde or ketone with fresh aldehyde or ketone. The aldehydes and ketones act not only as a reactant but also as a medium for transporting the water produced in the reaction. This method requires large excess of aldehyde (e.g. 4:1 molar ratio of aldehyde:alcohol) to be successful.
Reactive distillation is employed in U.S. Pat. No. 6,015,875 and U.S. Pat. No. 7,534,922 B2 to generate low boiling acetals. The authors make use of heterogeneous acids in the packing of the column and feed low boiling starting materials such as methanol, ethanol, formaldehyde, and acetaldehyde. The formed acetals are removed overhead above the distillation reaction zone and the co-product water is removed below the distillation reaction zone. This method limits the types of usable reactants to those producing materials that boil at a temperature lower than water.